Resin transfer molding has been used to manufacture composite or filament oriented structures of various sizes and shapes. Predominantly, this approach, which emerged from the textile industry, involves the assembly of dry, unimpregnated fibrous preforms for subsequent injection or infusion of a matrix resin, such as epoxy or the like. Historically, fabric is the starting point for most preforms because the fabric positions the reinforcing fibers in principal directions. Preform fabrics can be of any one of the conventional fabrics, such as a plain weave, satin, unidirectional or multidirectional knitted or bias weave fabrics. The fabric selection usually is dependent upon the final properties desired in the structure as well as the configuration of the manufactured part. As structural performances of a given component increases, the use of "non-crimped" fabric (that is, knitted and woven unidirectional fabrics) become more desirable because of the increase in tensile and compression performance. Increased performance is gained by eliminating the kinks within fiber bundles that result from the over-and-under construction of woven materials.
Preform fabrics may be of fiberglass, carbon and aramid fibers. Silicone carbide, aluminum oxide, boron, borsic, quartz and other fibers also are used in specific applications. As with all composite structures, selecting the reinforcing fiber for the preform depends on the end-use characteristics desired in the composite structure.
Generally, as defined in Johnson U.S. Pat. No. 4,762,740, dated Aug. 9, 1988, resin transfer molding is a closed mold, low pressure process applicable to the fabrication of complex, high performance composite articles of both large and small size. Several different resin transfer molding processes are known in the art, as set forth in somewhat detail in that patent. The process is differentiated from various other molding processes in that reinforcement fibrous material is placed separately into a molding tool cavity and then combined with resin within the mold cavity to form a fiber reinforced plastic composite product. Typically, a pre-shaped fiber reinforcement is positioned within a molding tool cavity and the molding tool then is closed. A feed line connects the closed molding cavity with a supply of liquid resin and the resin is pumped into the cavity where it impregnates and envelopes the fiber reinforcement and subsequently cures or is cured.
The desired mechanical and thermal performance of the composite structure is established during the preform design process. Preform assembly converts individual fabrics into a multilayered configuration specified by the composite designer. The fabric layers are assembled into the final configuration in a process analogous to a prepreg lamination operation. Fabric layers are placed in a predetermined orientation by rotating the principal axes of the fabric layers or by using a multidirectional fabric. The assembly process can be automated by using broad goods spreaders and robotic placement.
The final operation in preform assembly is the stitching process. Stitching mechanically fixes the final shape of the preform and constrains fiber movement during resin impregnation.
Whenever a stitching operation is to be performed, it is advantageous to work with a fiber preform that is free of resin or heavy binders because their presence limits the mobility of the fiber during needle penetration and results in a large number of fractured preform and thread fibers.
Resin injection fusion is a technique for impregnating preforms with hot-melt resin systems, which are resins that are solid at room temperature. These resins include typical aerospace-grade epoxies, bismaleimides, and polyimides.
Preforms are loaded into a holding fixture to stabilize the preform during the infusion process. Resin, in film form, is positioned uniformly onto the preform. Actual impregnation occurs in a heated vacuum chamber. The preform, with resin applied, is positioned in the chamber. Heat is then transmitted to the preform and resin by means of infrared radiation. A vacuum is applied during the impregnation cycle to remove entrained air and volatiles from the filmed resin. Preform impregnation occurs through capillary wetting of the preform as the viscosity of the resin decreases. During impregnation, the vacuum is cycled to prevent excessive resin bubbling and provide a mechanical pumping action to complement capillary wetting. At the end of this cycle, the resin is not cured.
In the resin film infusion/pressure molding process, the final sequence is the molding operation itself, which establishes the final shape of the composite structure, fixes the proportions of fiber and resin, cures the resin, and provides the composite structure with its designated mechanical and thermal properties.
In the resin transfer molding process, a thermosetting resin is injected into a cavity having the shape of the desired part. The cavity is filled with a dry fiber preform. The preform can include closed-cell cores and metal inserts, in addition to the fabric/mat materials. The process is typically used with low-viscosity fast-curing resins, such as polyester and epoxy, and chopped-strand or continuous mat reinforcement at a low fiber volume. However, the process has been demonstrated with higher fiber volumes of oriented material and higher-performance matrices, such as polyimides.
The preceding information is found in T. DeMint and H. Van Schoonevelt, Fiber Preforms and Resin Injection, in Engineered Materials Handbook, Volume 1, Composites, ASM International, 1987, p 529-532.
In general, designing preforms from layers of cloth fabric is a rather expensive proposition for use in resin transfer molding processes. First, purchasing the fabric itself is expensive. Then, the fabric or cloth layers must be laid-up onto a mandrel and, conventionally, stitched together into a multi-layer construction. Transporting the assembled layers or preform to a mold is a considerable problem. Still, such a preform does not provide the improved characteristics of a filament wound composite. On the other hand, continuous fibers can be purchased literally for pennies per linear length and easily laid-up by a simple winding process. If there was a simple solution to the transporting problem, filament wound preforms would be a vast improvement in the art.
This invention is directed to solving the above problems and satisfying the need for a new and improved filament wound resin transfer molding preform assembly process.